U.S. patent application number 13/317608 was filed with the patent office on 2012-05-03 for segmented thermoelectric module with bonded legs.
This patent application is currently assigned to BASF SE and Hi-Z Technology, Inc.. Invention is credited to Martin Gaertner, Frederick A. Leavitt, Panneerselvam Marudhachalam, John W. McCoy.
Application Number | 20120103381 13/317608 |
Document ID | / |
Family ID | 45995300 |
Filed Date | 2012-05-03 |
United States Patent
Application |
20120103381 |
Kind Code |
A1 |
Leavitt; Frederick A. ; et
al. |
May 3, 2012 |
Segmented thermoelectric module with bonded legs
Abstract
A segmented lead telluride egg-crate thermoelectric module. In
preferred embodiments N legs and P legs are segmented into at least
two segments. The segments are chosen for their figure of merit in
the various temperature ranges between the hot side and the cold
side of the module. In preferred embodiments a low-temperature
egg-crate, molded from a liquid crystal polymer material, having
very low thermal conductivity holds in place and provides
insulation and electrical connections for the thermoelectric N legs
and P legs at the cold side of the module. A castable ceramic
capable of operation at temperatures in excess of 500.degree. C. is
used to provide electrical insulation between the legs at the hot
side of the module.
Inventors: |
Leavitt; Frederick A.; (San
Diego, CA) ; McCoy; John W.; (San Diego, CA) ;
Marudhachalam; Panneerselvam; (Ludwigshafen, DE) ;
Gaertner; Martin; (Worms, DE) |
Assignee: |
BASF SE and Hi-Z Technology,
Inc.
|
Family ID: |
45995300 |
Appl. No.: |
13/317608 |
Filed: |
October 24, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12317170 |
Dec 19, 2008 |
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13317608 |
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12590653 |
Nov 12, 2009 |
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12317170 |
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Current U.S.
Class: |
136/238 |
Current CPC
Class: |
H01L 35/34 20130101;
H01L 35/30 20130101; H01L 35/32 20130101; F01N 5/025 20130101 |
Class at
Publication: |
136/238 |
International
Class: |
H01L 35/32 20060101
H01L035/32; H01L 35/20 20060101 H01L035/20 |
Claims
1. A thermoelectric module comprising: A. a two-part egg-crate for
holding in place and providing insulation and electrical
connections for a number of thermoelectric N-legs and P-legs,
wherein said egg-crate is comprised of: 1) a hot side part
comprised of a ceramic material capable of operation at
temperatures in excess of 500.degree. C. and 2) a cold side part
comprised of a polymeric material having very low thermal
conductivity. B. a plurality of segmented thermoelectric N-legs and
P-legs, each leg comprised of at least one PbTe segment and
positioned in said egg-crate, at least a portion of said legs being
electrically connected in series; wherein 1) each of at least a
plurality of said N-legs are comprised of a) a high-temperature
thermoelectric segment and b) a low-temperature thermoelectric
segment and 2) each of at least a plurality of said P-legs are
comprised of a) a high-temperature thermoelectric segment and b) a
low-temperature thermoelectric segment.
2. The thermoelectric module as in claim 1 wherein at least a
plurality of said low-temperature thermoelectric segments of said P
legs are comprised of BiTe.
3. The thermoelectric module as in claim 1 wherein at least a
plurality of said N and P legs are fabricated utilizing a
multi-cavity die having walls adapted for disassembly to permit
removal of sintered legs without damage.
4. The thermoelectric module as in claim 3 wherein the die is
comprised of molybdenum.
5. The thermoelectric module as in claim 1 wherein each of a
plurality of said N-legs also comprises at least one intermediate
temperature PbTe segment.
6. The thermoelectric module as in claim 1 wherein N legs are
comprised of two types of PbTe and the P legs are comprised of PbTe
on the hot side and Bi.sub.2Te.sub.3 on the cold side.
7. The thermoelectric module as in claim 1 wherein iron contacts
are vacuum hot pressed along with the thermoelectric materials to
create good compatible contact surfaces.
8. The thermoelectric module as in claim 1 where interfaces between
the materials are graduated in composition to improve
performance.
9. The thermoelectric module as in claim 1 where special techniques
are utilized in the leg fabrication process to assure fines are
removed.
10. The thermoelectric module as in claim 1 wherein a thin copper
binding layer is also added on top of the iron at the hot side of
the legs and hot pressed into the leg material.
11. The thermoelectric module as in claim 1 wherein dual egg-crate
is provided with high temperature ceramic used at the hot side and
low thermal conductivity moldable thermo plastic is used for the
cold side.
12. The thermoelectric module as in claim 1 wherein a molybdenum
sulfide lubricant is applied to die surfaces in the fabrication of
the legs to minimize or eliminate leg damage during removal from
the die.
13. The thermoelectric module as in claim 1 wherein the modules are
hermetically sealed either individually or as a part of
thermoelectric systems.
14. The thermoelectric module as in claim 1 wherein each of a
plurality of said P-legs also comprises at least one intermediate
temperature PbTe segment.
15. The thermoelectric module as in claim 1 wherein said plurality
of said lead-telluride thermoelectric N-legs and P-legs are
electrically connected with one or more metals thermally sprayed on
at least one side of the module defining a cold side.
16. The thermoelectric module as in claim 15 wherein said one or
more metals is zinc.
17. The thermoelectric module as in claim 16 wherein said one or
more metals is molybdenum and aluminum.
18. The thermoelectric module as in claim 1 wherein said ceramic
material is zirconium oxide and said polymeric material is in the
form of a liquid crystal polymer resin.
19. The thermoelectric module as in claim 1 wherein a plurality of
said thermoelectric legs comprise fine micron/nano-sized
grains.
20. The thermoelectric module as in claim 1 wherein each N-leg and
P-leg of at least a plurality of pairs of said thermoelectric
N-legs and P-legs are electrically connected utilizing an iron
shoe.
21. The thermoelectric module as in claim 1 wherein each N-leg and
P-leg of at least a plurality of pairs of said thermoelectric
N-legs and P-legs are electrically connected utilizing a copper
strip.
22. The thermoelectric module as in claim 15 wherein the egg-crate
walls separating the n-legs from the p-legs are adapted to contact
the hot conductor so that tellurium vapor is restrained from
migrating to the n-leg.
23. The thermoelectric module as in claim 7 wherein at least a
plurality of the P-legs comprise a thin layer of PbSnMnTe at their
hot sides.
24. The thermoelectric module as in claim 7 wherein at least a
plurality of the P-legs comprise a thin layer of PbSnTe at their
hot sides.
25. The thermoelectric module as in claim 7 wherein at least a
plurality of the P-legs comprise a thin layer of SnTe at their hot
sides.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present invention is a continuation-in-part of Ser. No.
12/317,170 filed Dec. 19, 2008 and Ser. No. 12/590,653, filed Nov.
12, 2009, both of which are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to thermoelectric modules and
especially to mid temperature to high temperature thermoelectric
modules.
BACKGROUND OF THE INVENTION
Thermoelectric Materials
[0003] The Seebeck coefficient of a thermoelectric material is
defined as the open circuit voltage produced between two points on
a conductor, where a uniform temperature difference of 1 K exists
between those points.
[0004] The figure-of-merit of a thermoelectric material is defined
as:
Z = .alpha. 2 .sigma. .kappa. , ##EQU00001##
where .alpha. is the Seebeck coefficient of the material, .sigma.
is the electrical conductivity of the material and .kappa. is the
total thermal conductivity of the material.
[0005] A large number of semiconductor materials were being
investigated by the late 1950's and early 1960's, several of which
emerged with Z values significantly higher than in metals or metal
alloys. As expected no single compound semiconductor evolved that
exhibited a uniformly high figure-of-merit over a wide temperature
range, so research focused on developing materials with high
figure-of-merit values over relatively narrow temperature ranges.
Of the great number of materials investigated, those based on
bismuth telluride and lead telluride alloys emerged as the best for
operating in various temperature ranges up to 600.degree. C. Much
research has been done to improve the thermoelectric properties of
the above thermoelectric materials. For example n-type
Bi.sub.2Te.sub.3 typically contains 5 to 15 percent
Bi.sub.2Se.sub.3 and p-type Bi.sub.2Te.sub.3 typically contains 75
to 80 Mol percent Sb.sub.2Te.sub.3. Lead telluride is typically
doped with Na and enriched in Te for P type behavior and for N type
behavior the lead telluride is typically doped with iodine and
enriched in Pb.
Standard Designations
[0006] The temperature at which a thermoelectric alloy is most
efficient can usually be shifted to higher or lower temperatures by
varying the doping levels and additives. Some of the more common
variations with PbTe alloys are designated in the thermoelectric
industry as 3N and 2N for N type and 2P and 3P for P type. An in
depth discussion of PbTe alloys and their respective doping
compositions is given in the book, Thermoelectric Materials and
Devices, edited by Cadoff and Miller, Chapter 10 "Lead Telluride
Alloys and Junctions." For further understanding of
Bi.sub.2Te.sub.3 based alloys and their doping, see Chapter 9 of
the above book and two books edited by D. M. Rowe "CRC Handbook of
Thermoelectrics, especially Chapter 19 and Thermoelectrics Handbook
"Macro to Nano, Chapter 27. In this specification and in the claims
the term PbTe is meant to include any lead and tellurium
semi-conductor alloy when both the lead and tellurium Mol
percentage is greater than 20 percent. This includes intrinsic or
doped N or P type PbTe, PbSnMnTe and PbSnTe alloys, PbTe doped with
Thallium, or AgTe.sub.2.
Temperature Ranges for Best Performance
[0007] Thermoelectric materials can be divided into three
categories: low, mid-range and high temperature.
Low Temperature
[0008] Commercially available low-temperature materials normally
include Bi.sub.2Te.sub.3 alloys. When operated in air, these
materials can not exceed 250.degree. C. on a continuous basis
without severe deterioration in performance. These alloys are
mainly used for cooling although there are a number of waste heat
recovery applications based on these Bi.sub.2Te.sub.3 alloys. When
used as a power source, Bi.sub.2Te.sub.3 alloys rarely exceed 5%
efficiency.
Mid-Range Temperature
[0009] Mid-range materials are normally based on the use of lead
telluride, PbTe. PbTe can operate up to about 560.degree. C.
Thermoelectric legs comprised primarily of the TAGS group of
materials (tellurium, antimony, germanium and silver) provide good
performance at about 450.degree. C. Some cobalt based alloys
(referred to as skutterudites) are being investigated that also
fall into this category but they exhibit high vaporization rates
which must be contained for long life. All mid-range thermoelectric
alloys known to Applicants will oxidize in air and must be
hermetically sealed. Prior art PbTe alloys rarely exceed about 7
percent efficiency. A large number of doping materials are
currently being proposed for improvements in performance for all of
these mid-range materials.
High-Temperature--Primarily for Space Applications
[0010] High-temperature thermoelectric materials are normally based
on SiGe and Zintl alloys and can operate near 1,000.degree. C.
Modules based on these alloys are difficult to fabricate, expensive
and are normally used only in space applications. These prior art
high temperature materials can achieve efficiencies as high as 9
percent in some applications, but to date commercial application of
these modules has been rare.
Segmented Legs
[0011] Segmented thermoelectric legs with mid-temperature to
high-temperature materials on the hot side of the leg and a low
temperature material on the cold side of the legs can significantly
improve performance.
[0012] Some of the higher temperature thermoelectric materials tend
to experience high free vaporization rates (such as 50% loss in 300
hours). These modules can be sealed in a metal package referred to
as a can. The process is called canning. Alternately, one
fabricator has contained the material in aerogel insulation in an
attempt to suppress the evaporation. In another vapor suppression
approach, the sample was coated with 10 .mu.m of titanium. Metal
coatings can produce electrical and thermal shorting.
Thermoelectric Modules
[0013] Thermoelectric power production is typically accomplished
with a number of thermoelectric modules sandwiched between a hot
surface and a cold surface. These modules produce electricity
directly from a temperature differential utilizing the
thermoelectric effect. The modules typically include P-type
thermoelectric semiconductor elements and N-type thermoelectric
semiconductor elements. These thermoelectric elements are called N
legs and P legs. The effect is that a voltage differential of a few
millivolts is created in each leg in the presence of a temperature
difference of a few hundred degrees. Since the voltage differential
is small, many of these legs (such as about 100 legs in each
module) are typically positioned side-by-side between the hot
surface and the cold surface but are connected electrically in
series to produce open circuit potentials of a few volts and power
output in the range of a few watts per module. A large number of
these modules can be combined to produce power in the kilowatt
range from a heat source such as the exhaust system of a truck.
Thermoelectric modules are well suited to recover energy from a
variety of waste heat applications because they are:
TABLE-US-00001 Small Easily scaled up or down Solid state Highly
reliable Silent Potentially cost effective
Hi-Z Prior Art Bismuth Telluride Molded Egg-Crate Modules
[0014] For example Hi-Z Technology, Inc., with offices in San Diego
Calif., offers a Model HZ-14 thermoelectric bismuth telluride
thermoelectric module designed to produce about 14 watts at a load
potential of 1.66 volts with a 200.degree. C. temperature
differential. Its open circuit potential is about 3.5 volts. The
module contains 49 N legs and 49 P legs connected electrically in
series. It is a 0.5 cm thick square module with 6.27 cm sides. The
legs are P-type and N-type bismuth telluride semiconductor legs and
are positioned in an egg-crate type structure that insulates the
legs from each other except where they are intentionally connected
in series at the top and bottom surfaces of the module. That
egg-crate structure which has spaces for the 98 active legs is
described in U.S. Pat. No. 5,875,098 which is hereby incorporated
herein by reference. The egg-crate is injection molded in a process
described in detail in the '098 patent. This egg-crate has greatly
reduced the fabrication cost of these modules and improved
performance for reasons explained in the patent. FIG. 1 is a
drawing of the egg-crate and FIG. 2 is a cross sectional drawing of
a portion of the egg-crate showing how the P-legs and N-legs are
connected in series in the egg-crate. The curved arrows e show the
direction of electron flow through top conductors 2, N legs 4,
bottom conductors 6, and P legs 8 in this portion 10 of the module.
Insulating walls 14 keep the electrons flowing in the desired
series circuit. Other Bi.sub.2Te.sub.3 thermoelectric modules that
are available at Hi-Z are designed to produce 2.5 watts, 9 watts,
14 watts and 20 watts at the 200.degree. C. temperature
differential as explained above. The term bismuth telluride is
often used in the thermoelectric industry to refer to all
combinations of Bi.sub.2Te.sub.3, Bi.sub.2Se.sub.3,
Sb.sub.2Te.sub.3 and Sb.sub.2Se.sub.3. (This apparently is because
antimony is chemically similar to bismuth and selenium is
chemically similar to tellurium.) In this document where the term
Bi.sub.2Te.sub.3 is used, it means any combination of
Bi.sub.2Te.sub.3, Bi.sub.2Se.sub.3, Sb.sub.2Te.sub.3 and
Sb.sub.2Se.sub.3.
Temperature Limitations
[0015] The egg-crates for the above described Bi.sub.2Te.sub.3
modules are injection molded using a thermoplastic supplied by
Dupont under the trade name "Zenite". Zenite melts at a temperature
of about 350.degree. C. The thermoelectric properties of
Bi.sub.2Te.sub.3 peak at about 100.degree. C. and are greatly
reduced at about 250.degree. C. For both of these reasons, uses of
these modules are limited to applications where the hot side
temperatures are lower than about 250.degree. C.
Thermoelectric Materials
Figures of Merit
Thermoelectric Materials
[0016] Many different thermoelectric materials are available. These
include bismuth telluride, lead telluride, silicon germanium,
silicon carbide, boron carbide and many others. In these materials
relative abundance and doping ranges can make huge differences in
the thermoelectric properties. Much experimental data regarding
these materials and their properties is available in the
thermoelectric literature such as the CRC Handbook referenced
above. Each of these materials is rated by their "figure of merit"
(Z) which in all cases is very temperature dependent. Despite the
fact that there exists a great need for non-polluting electric
power and the fact that there exists a very wide variety of
un-tapped heat sources; thermoelectric electric power generation in
the United States and other countries is minimal as compared to
other sources of electric power. The reason primarily is that
thermoelectric efficiencies are typically low compared to other
technologies for electric power generation and the cost of
thermoelectric systems per watt generated is relatively high
compared to other power generating sources. Generally the
efficiencies of thermoelectric power generating systems are in the
range of about 5 percent.
Lead Telluride Modules
[0017] Lead telluride thermoelectric modules are also known in the
prior art. A prior art example is the PbTe thermoelectric module
described in U.S. Pat. No. 4,611,089 issued many years ago to two
of the present inventors. This patent is hereby incorporated herein
by reference. That module utilized lead telluride thermoelectric
alloys with an excess of lead for the N legs and lead telluride
with an excess of tellurium for the P legs. Performance can be
improved with doping using known techniques. The thermoelectric
properties of heavily doped lead telluride thermoelectric alloys
peak in the range of about 425.degree. C. The egg-crate for the
module described in the above patent was fabricated using a
technique similar to the technique used many years ago for making
chicken egg crates using cardboard spacers. For the thermoelectric
egg-crate the spacers were mica which was selected for its
electrical insulating properties at high temperatures. Mica,
however, is marginal in strength and cracks easily. A more rugged
high-temperature material is needed.
[0018] FIG. 3 is a drawing from the U.S. Pat. No. 4,611,089 showing
a blow-up of the module described in that patent. The egg-crate
included a first set of parallel spacers 46a to 46k and a second
set of spacers 48a to 48i. The N legs are shown at 52 and the P
legs are shown at 54. The module included hot side conductors 56
and cold side conductors 58 to connect the legs in series as in the
Bi.sub.2Te.sub.3 module described above.
[0019] That lead telluride module was suited for operation in
temperature ranges in excess of 500.degree. C. But the cost of
fabrication of this prior art module is greatly in excess of the
bismuth telluride module described above. Also, after a period of
operation of about 1000 hours some evaporation of the P legs and
the N legs at the hot side would produce cross contamination of all
of the legs which would result in degraded performance. Prior art
thermoelectric modules have required special compression techniques
applied to the modules to assure good electrical contacts with
respect to the various segments of the thermoelectric legs.
[0020] What is needed is a fully bonded, low-cost, mid to
high-temperature, high-efficiency thermoelectric module designed
for operation at hot side temperatures in the range of 500.degree.
C. or higher preferably with thermoelectric properties
substantially in excess of prior art high-temperature
thermoelectric modules.
SUMMARY OF THE INVENTION
[0021] The present invention provides a fully bonded, segmented,
long-life, low-cost, mid-temperature to high-temperature,
high-efficiency thermoelectric module. Preferred embodiments
include a multi-segment, egg-crate module with N legs and P legs
that are segmented into at least two segments of thermoelectric
materials. In preferred embodiments the segments are chosen for
their figure of merit in the various temperature ranges between the
hot side and the cold side of the module. In preferred embodiments
a low-temperature egg-crate, molded from a liquid crystal polymer
material, having relatively very low thermal conductivity holds the
legs in place and provides insulation and permits electrical
connections for the thermoelectric N legs and P legs to be
efficiently applied at the cold side of the module. A castable
ceramic capable of operation at temperatures in excess of
500.degree. C. is used to provide electrical insulation between the
legs at the hot side of the module. In preferred embodiments the
high-temperature ceramic is Resbond 989 or Resbond 908 which is
available as a high-temperature, general purpose ceramic adhesive
from Cotronix Corporation, and the liquid crystal polymer material
is Zenite available from DuPont in the form of a liquid crystal
polymer resin. In preferred embodiments the module is sealed in an
insulating capsule or a number of modules are sealed together in a
thermoelectric generator. All of the parts of the module a solidly
bonded together is the preferred module fabrication process so that
external pressure is not necessary to assure good contacts within
the module.
[0022] In a preferred embodiment the N legs are comprised of two
types of PbTe and the P legs are comprised of PbTe on the hot side
and BiTe on the cold side. To fabricate the legs for this preferred
embodiment, iron contacts are vacuum hot pressed along with the
thermoelectric materials to create good compatible contact
surfaces. Interfaces between the materials are graduated to improve
performance. Special techniques are utilized to assure fines are
removed. A thin copper binding layer is also added on top of the
iron at the hot side of the legs and hot pressed into the leg
material. A dual egg-crate is provided with high temperature
ceramic used at the hot side and low thermal conductivity moldable
thermo plastic is used for the cold side. To fabricate the legs a
molybdenum sulfide lubricant is applied to all surfaces of the hot
pressing die and plungers to minimize or eliminate leg damage
during removal from the die. Tungsten or molybdenum disulfide could
also be used as the lubricant.
[0023] In preferred embodiments the hot and middle segments for the
N legs are two or three types of lead telluride thermoelectric
material (3N and/or 2N) and the low-temperature material is bismuth
telluride. The hot and middle segments for the P legs are also lead
telluride (3P and/or 2P, respectively). And the low-temperature
material again is bismuth telluride. In preferred embodiments
low-temperature contacts are provided by thermally sprayed a
thermal spray metalizing process using a molybdenum bonding layer
and an aluminum top layer or a single layer of zinc, either of
which provides excellent electrical contacts between the N and P
legs.
[0024] The two or three segment legs are produced using a vacuum
hot pressed powder metallurgy process in which the leg materials
are added to a multi-cavity molybdenum die and hot pressed
simultaneously. First a thin layer of iron powder is inserted into
the cavity. Then the two or three thermoelectric materials are
sequentially added. Iron metal powder is then mixed with lead
telluride powder and added at the hot end of each of the P and N
legs to provide a one millimeter thick graded layer of PbTe and
iron powder. On top of the graded layer a thin layer of 100 percent
iron metal layer of powder is added to form the top of the legs.
Then the leg powders are then hot pressed at 7,000 psi and
600.degree. C. The iron layer at the top of the legs chemically
isolates the PbTe from a copper layer which is added during a
centering step following the hot press. The purpose of the thin
copper layer is to aid in the bonding of a copper conductor which
connects the N and P legs at the hot side electrically in
series.
[0025] The hot end of each leg is capped with a copper segment
added to enable solid-state diffusion monding to a copper "jumper"
or "hotshoe" placed above it that bridges between pairs of legs in
the module providing electrical connections. The copper segment is
formed from a copper foil, and is bonded and integrate into the
rest of the leg during the same hot pressing and sintering
operation that densifies the PbTe and Bi.sub.2Te.sub.3 powders. A
reaction-barrier layer of iron is disposed between the copper and
PbTe segments to prevent diffusion of copper into PbTe and the
consequent formation of CuTe. The iron layer is formed from iron
powder densified during the hot-pressing and sintering
operation.
Special Cold Side Contacts
[0026] With a Bi.sub.2Te.sub.3 segment on the cold side of the PbTe
leg it is possible to use Applicants' employer's standard prior art
Bi.sub.2Te.sub.3 contacting methods as described in U.S. Pat. No.
5,856,200, especially FIGS. 19A and 19B and related text, which is
incorporated by reference herein. This method is a method of
forming contacts to Bi.sub.2Te.sub.3 using metallic thermal
spraying process. The resultant cold side contact is firmly bonded
to the legs and eliminates the need to make numerous individual
electrical connections. Preferred metallization schemes include:
(1) pure zinc, (2) a two-layer system using pure molybdenum as a
bond coating and pure aluminum as the electrical and thermal
conductor layer.
Multi-Cavity Molybdenum Die
[0027] In preferred embodiments the legs are formed by placing the
appropriate layers of the powder in a multi-cavity molybdenum die
with each cavity forming the desired geometry. To allow the parts
to be removed without damaging them the die is designed to be
disassembled in such a way that the individual legs can be removed
with out being subject to undue forces.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a drawing of a prior art egg-crate for a
thermoelectric module.
[0029] FIG. 2 is a drawing of a portion of a module with the FIG. 1
egg-crate.
[0030] FIGS. 3 and 4 are prior art blown-up drawing of a prior art
lead telluride thermoelectric module.
[0031] FIG. 5A is a prior art drawing showing an application used
to generate electricity from the exhaust gas of a truck.
[0032] FIG. 5B shows an encapsulated module.
[0033] FIGS. 6A and 6B are graphs showing figures of merit for 2N,
3N, 2P and 3P lead telluride thermoelectric material and N and P
bismuth telluride material.
[0034] FIG. 7 is a drawing of a P leg in a preferred
embodiment.
[0035] FIG. 8 is a drawing of an N leg in a preferred
embodiment.
[0036] FIG. 9 shows features of a preferred module in accordance
with the present invention.
[0037] FIGS. 10A, 10B and 10C are drawing showing how the legs are
assembled for hot pressing.
[0038] FIG. 11 shows a design of a thermoelectric generator for
utilizing the modules for electricity production.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Lead Telluride and Bismuth Telluride
[0039] Lead Telluride thermoelectric materials permit the design of
thermoelectric modules that can operate more efficiently at higher
temperatures as compared to modules based on bismuth telluride
alloys. Unlike bismuth telluride however, lead telluride is less
ductile and cracks more readily than does bismuth telluride. This
makes it difficult to build a bonded module and so lead telluride
modules are typically made by assembling many individual components
that are subsequently held in compression with pressures as high as
1,000 psi. The high compressive force causes a bond to form between
the lead telluride and the contact materials at operating
temperatures, but these bonds tend to break when the modules cools.
The present invention provides a method of forming permanent bonds
on both the hot and cold side of the module which eliminates the
need for high compressive forces and permits thermally cycling
without the substantial risk of breakage. Details are provided in
the sections that follow.
[0040] Bismuth telluride works best at temperatures below about
250.degree. C. Bismuth telluride thermoelectric material is
available form Marlow Industries with offices in Dallas, Tex. The
bismuth telluride segments are preferably doped with 0.1 Mol
percent iodoform (CHl.sub.3) to create the low temperature N-type
material and 0.1 part per million Pb to create lower temperature
P-type material. Several successful methods are available for
fabrication of PbTe materials and are described in "Lead Telluride
Alloys and Junctions" of Thermoelectric Materials and Devices,
Cadoff and Miller, published by Reinhold Publishing Corporation of
New York.
[0041] Several successful methods are available for making PbTe
thermoelectric materials. In a preferred technique appropriate
amounts of raw materials are weighed out, mixed and sealed in a
quartz tube with an inert atmosphere such as argon. The tube is
then heated to 900.degree. C. for 90 minutes and allowed to cool.
The resultant ingot is then ground to a -20 mesh powder and
lubricated with 0.15 weight percent graphite powder. The powder can
then be processed into thermoelectric legs as described in various
sections of this specification.
[0042] Performance of the lead telluride thermoelectric material at
various temperatures depends on the ratio of lead to telluride and
also additional materials that can be added to improve performance.
Applicants refer to various combinations as 2P, 3P, 2N and 3N lead
telluride. Performance of these lead telluride materials as a
function of temperature are shown in FIGS. 6A and 6B. In this
specification these lead telluride combinations are defined as
follows:
TABLE-US-00002 2P lead 49.5 mol % Tellurium 50.25 mol % Na2Te 0.25
mol % 3P lead 19.74 mol % Tellurium 49.61 mol % Tin 26.93 mol %
Manganese 3.47 mol % Na2Te 0.25 mol % 2N lead 50.1 mol % Tellurium
49.98 mol % PbI2 0.014 mol % 3N lead 50.00 mol % Tellurium 49.972
mol % PbI2 0.028 mol %
First Preferred Embodiment
[0043] A first preferred embodiment can be described by reference
to FIGS. 7 through 10C. FIG. 7 shows a P leg of this first
preferred embodiment. At the top is a copper layer 90 on the hot
end to aid in bonding to a copper conductor. Below copper layer 90
is an iron layer 92 to and a mixed layer 94 of 25 percent Fe and 75
percent P-type PbTe to chemically insulate the PbTe layer 96 from
the copper layer 90. Below the PbTe layer 96 is a Bi.sub.2Te.sub.3
layer 98 and at the bottom is a thin layer of Fe 100 to enhance
bonds with a later to be provided sprayed zinc connections. The
thermoelectric material for layers 96 and 98 are chosen to provide
high thermoelectric efficiencies at both the higher and lower
temperature regions of the P leg. FIG. 8 shows a similar design for
the N legs. At the top is a copper layer 102 and Fe and mixed
layers 104 and 106 as for the P leg. The 3N layer 108 is a layer of
PbTe designed for high efficiencies at temperatures close to
500.degree. C. and the 2N 110 is a layer of PbTe designed for high
efficiencies at temperatures in the range below about 450.degree.
C. At the bottom of the N leg 115 is a mixed layer 112 similar to
the one at the top and a layer of iron 114 similar to the iron
layer at the top.
[0044] FIG. 9 shows a section of four legs of a completed module
according to this first preferred embodiment showing how the module
is constructed. The drawing shows two n legs 115 and two P legs
101. The hot segment 108 of the N leg has been doped to be more
efficient at higher temperatures and the cold segment of the N leg
has been doped for higher efficiencies at lower temperatures. Iron
powder 104 and has been placed at both ends of the N and P legs to
act as a diffusion barrier between the thermoelectric materials and
the copper and zinc contacts.
[0045] A liquid polymer egg-crate 116 holds the legs in place and
defines the pattern for the cold side of the module. Copper wire
118 is soldered into the Zn cold junctions 121 to provide the high
and low voltage contacts for the module. This drawing shows only
portions of four legs. A typical module would contain about 100
legs. Copper layers 90 and 102 provide a surface suitable for
diffusion bonding to copper junctions 120 at the hot side of the
module connecting all of the legs electrically in series. Empty
spaces on the hot side of the module are filled with castable
ceramic to inhibit sublimation of PbTe. The ceramic also assures
low thermal conductivity and adds strength to the module.
Iron Contact Surfaces
[0046] Very few materials are compatible with PbTe. Many materials
that are compatible are not ideal due to their thermal expansion
coefficients, electrical conductivity, formability and cost. One of
the few materials that is compatible with PbTe is iron so a layer
of Fe about one millimeter thick is formed on the hot end of the
Bi2Te3-PbTe legs. In preferred embodiments Applicants also add a
thin layer of iron mixed with 10 percent Bi.sub.2Te.sub.3 to act as
a binder and strengthen the segment at the cold end of the legs.
Once in place the contact materials can then be bonded to the Fe
surface without worrying about compromising the PbTe.
Graduated Interface
[0047] Fe has a thermal expansion coefficient of
11.8.times.10.sup.-6 K.sup.-1 and PbTe has a thermal expansion
coefficient of about 18.times.10.sup.-6 K.sup.-1. This difference
causes a stress to form at the Fe/PbTe interface when the leg is
heated to its operating temperature of 575.degree. C. The stress
that forms will occasionally cause small micro-cracks to form in
the P leg since it is less ductile than the N leg. The micro-cracks
will sometimes show up as a high resistance in the finished legs.
To alleviate the stresses caused by the difference in thermal
expansion a thin layer of transition material about 1 mm thick is
placed between the Fe layer and the PbTe leg. The transition
material consists of 75% PbTe and 25% Fe powders by weight. The
PbTe material is ground to a -40 mesh powder and the Fe is a -325
mesh powder. The two powders are mixed by tumbling for one hour.
This is often referred to as a "functionally-graded interface".
Vacuum Hot Pressed Legs
[0048] To form a strong, low resistance bond (less than 10
.mu..OMEGA.) between the Fe layer, the graduated layer and the PbTe
layers it is very desirable to simultaneously press the layers
together while in a vacuum and heat to 600.degree. C.). Acceptable
results are obtained with a 10 .mu.m vacuum, 7,000 psi and
600.degree. C. held for one hour. This preferably can also be done
at 500.degree. C. and 6,000 psi for two hours or more.
Multi-Cavity Molybdenum Die
[0049] The legs are formed by placing the appropriate layers of the
powder in a multi-cavity die with each cavity forming the desired
geometry. To allow the parts to be removed without damaging them
the die is designed to be disassembled in such a way that the
individual legs can be removed with out being subject to undue
forces. It is necessary for the die to be fabricated from a
material that is compatible with the PbTe and still capable of
withstanding high stresses at 600.degree. C. Some candidate
materials are high-temperature iron nickel chrome alloys such as
A286, or from pure Molybdenum metal, or from a tungsten
carbide/cobalt cermet or from graphite, or from H13 tool steel. In
preferred embodiments Applicants have fabricated a multi-cavity die
from molybdenum. FIGS. 10A, 160 and 10C show the components of the
die and how it is assembled and utilized in the hot pressing
process.
[0050] As shown in FIG. 10A the molybdenum die consists of walls
201 and die ring 202. When fully assembled as shown in FIGS. 10A
and 10B, 121 cavities are created in the assembly. The dimensions
of each of the cavities are 5 mm.times.5 mm.times.7 mm. To keep the
die walls from falling out the bottom of die assembly 128, a bottom
plate 130 with 121 matching pin holes is positioned below the die
assembly. All parts of the assembly are sprayed with tungsten
disulfide to assure good lubrication and no sticking of the leg
material to the assembly parts. The dies fabricated from Mo have
demonstrated good results. Molybdenum is much stronger and tougher
than graphite, does not deform or creep after prolonged exposure to
stress at elevated temperature, and it does not suffer "galling" as
does A286 or any stainless steel.
Creating a Segmented Leg
[0051] To form a leg that consists of multiple segments or layers a
bottom punch is first inserted into the die described above. The
punch is held at the desired depth below the surface of the die by
a set of pins inserted into the die cavity from the bottom of the
die. The pins are held in position by a pin plate into which they
have been inserted. The desired powder is then placed into the die
cavities and scraped off so as to be level with the surface of the
die. A spacer is then placed between the bottom of the die and the
plate that holds the pins that support the bottom punch. With the
spacer in between the pin plate and the die the bottom punch and
powder already placed into the die are allowed to fall to a lower
level. The first charge of powder is then compressed so that the
space between the top of the powder and the top of the die is the
desired distance and the empty space in the die is filled with the
second powder that is to form the second segment of the leg. This
process is repeated for each of the desired segments. Once the die
is filled with the correct amount of powder in the correct
positions the bottom die is allowed to drop to the bottom of the
die and a top punch is inserted. Spacers are used to allow the
bottom punch to extend below the bottom of the die until the die is
placed into the press. Once a small amount of pressure is applied
to the loaded die to keep the die and punches in position the
spacers are removed. It is desirable to have both the bottom and
the top punches extending outside the die so that pressure is
applied from both the top and bottom punches. This is referred to
as a double acting press and results in more homogeneous density
within the leg. The length of the punches and the volume of powder
placed into the die is designed to reach maximum density when the
surface of the punch comes level with the surface of the die. This
results in legs that are all the same length thereby minimizing
variation in the length of the legs. The precise procedures for
fabricating N legs and P legs for a preferred embodiment shown in
FIGS. 7, 8 and 9 are described below with reference to FIG.
10C.
General Procedure for Loading Powders
[0052] In order to provide the desired volume of powder of each
layer of the thermoelectric legs, lower punches 132 are inserted
into the die and held at desired heights by punch pins 134 fastened
to pin plate 136. The pins are inserted through the holes in bottom
plate 130. The pins and the plate are standard components of
commercially available punch presses. The pins are adjusted to an
appropriate position to create a cavity of a desired height in the
die assembly for a layer of the thermoelectric legs being
fabricated. Then powder for that layer is placed on top of the
assembly and allowed to fill the created cavity. Then excess powder
is scrapped off. The pins are then lowered leaving a new cavity in
the assembly. Then powder for the next layer is placed on top of
the assembly and the process is repeated until the powder for all
of the layers of the legs have been inserted in the die assembly.
Then pins are lowered and the top punches 207 are placed in the
cavities after which the pin plate is removed. The loaded die
assembly is then placed in a vacuum hot press.
N Legs
[0053] Table I describes the step-by-step process of loading the
die assembly for the N legs. The materials used are: PbTe powder,
Fe powder, (99.9%-220+325 mesh) mixed Cu foil and WS.sub.2
lubricant. The mixed powder is 75 percent 2N PbTe and 25 percent
Fe. Punch positions are adjusted with spacers placed below the pen
plate 136. The height of the spacers are provided in parentheses in
the table.
TABLE-US-00003 TABLE I Die Assembly Loading Process for N Legs Step
1 Assemble die, coat all parts with WS.sub.2 and insert walls into
ring. Step 2 Place a bottom punch in each cavity. Step 3 Drop the
punches 2.0 mm and fill all cavities with the mixed Fe powder. (1.5
mm spacer) Step 4 Drop the punches an additional 1.0 mm and fill
with 0.75, 2N/0.25Fe mixed powder. (2.5 mm spacer) Step 5 Drop the
punches 9.0 mm and push them to the bottom of the die. Fill with 2N
powder. (11.5 mm spacer) Step 6 Compact the powder 1.5 mm and fill
with 0.75, 2N/0.25Fe mixed powder. (11.5 mm spacer) Step 7 Compact
the powder 2.0 mm and fill with mixed Fe powder. (11.5 mm spacer)
Step 8 Compress the powder and place a 4.5 .times. 4.5 .times. 0.5
mm piece of copper on top the Fe powder. (14.0 mm spacer) Step 9
Force the punches (and the powder) to the bottom of the die taking
care to ensure no Fe powder ends up on top of the copper. Use a
lightly magnetic set of tweezers to remove any Fe that does get on
top of the copper. Be sure that the Fe underneath the Cu is not
disturbed. Add the top punches.
Compressing the Powder for the N Legs
[0054] After the die assembly has been loaded with powder, the
loaded die assembly is loaded into a hot press and 1,000 pounds of
force is applied. A thermocouple is then inserted into the die
assembly. While cold, a vacuum of about 10 microns is pulled and
the hot press is backfilled with argon mixed with 5 percent H.sub.2
to a 10 inch vacuum. These two steps are repeated two more times.
The die is heated to 600.degree. C. A vacuum of less than 200
microns is pulled and the chamber is backfilled with the argon
mixture to 10 inches. These two steps are repeated two more times.
A vacuum of about 10 microns is pulled. The force on each leg is
then increased to about 250 pounds. For the 121 legs this will
require of total force of about 30,250 pounds. The force is held
for 15 minutes. The chamber is then backfilled with the argon
mixture to 10 inches vacuum. The temperature and force is held at
600.degree. C. and 250 pounds per leg (about 6,000 psi) for one
hour. The load is then reduced to 500 pounds and the chamber is
cooled to below 50.degree. C. and the parts are removed.
P Legs
[0055] Table 2 describes the step-by-step process of loading the
die assembly for the P legs. The materials used are: PbTe powder,
Bi.sub.2Te.sub.3 powder ground to -45 mesh, Fe powder,
99.9%-220+325 mesh, mixed (75 percent Pb and 25 percent Fe) and
WS.sub.2 lubricant. Punch positions are adjusted with spacers
placed below the pin plate 206. The height of the spacers is
provided in parentheses in the table.
TABLE-US-00004 TABLE 2 Die Assembly Loading Process for P Legs Step
1 Assemble die, coat all parts with WS.sub.2 and insert walls into
ring. Step 2 Place a bottom punch in each cavity. Step 3 Drop
punches 2.0 mm and fill all cavities with the mixed Fe powder mixed
with 10% Bi.sub.2Te.sub.3 powder. (1.5 mm spacer) Step 4 Drop
punches an additional 5.5 mm and fill with Bi.sub.2Te.sub.3 powder.
(7.0 mm spacer) Step 5 Compress the powder and fill with BP powder.
(10.0 mm spacer) Step 6 Compress the powder and fill with
0.75BP/0.25Fe mixed powder. (11.0 mm spacer) Step 7 Compact the
powder 3.0 mm and fill with mixed Fe powder. (13.5 mm spacer) Step
8 Compress the powder and place a 4.5 .times. 4.5 .times. 0.5 mm
piece of copper on top of the Fe powder. (14.0 mm spacer) Step 9
Force the punches (and the powder) to the bottom of the die taking
care to ensure no Fe powder ends up on top of the copper. Use a
lightly magnetic set of tweezers to remove any Fe that does get on
top of the copper. Be sure that the Fe underneath the Cu is not
disturbed. Add the top punches.
Compressing the Powders for the P Legs
[0056] The process for compressing the powders for the P legs is
the same as the process for the N legs except the peak temperature
is limited to 500.degree. C.
Removal of Fines
[0057] When using the die to fabricate thermoelectric legs it is
necessary to remove the very fine portion of the powders. The very
fine materials that are less than 325 mesh are able to migrate in
between the various pieces of the assembled die and contaminate the
PbTe powders. A suitable particle size distribution is -140+325
mesh.
Copper Bonding Layer
[0058] The thermoelectric elements that result from the layering
process are then assembled into an eggcrate structure that holds
the legs in a checker board pattern so that all N legs have only P
leg neighbours and all P legs have only N leg neighbors. The hot
end of a P leg is then electrically connected to the hot end of the
adjacent N leg. To make a good low resistance connection it is
preferable to use a copper conductor but while copper will bond to
iron it does not form a strong enough bond to withstand normal
handling and will often fail due to thermal expansion differences
alone. If a copper bond layer is built into the hot end of the leg
then copper can easily be diffusion bonded to the leg and a strong
low resistance bond can be formed. The copper bonding layer is
formed by sand blasting a piece of copper foil 0.020'' thick and
placing it in the die on top of the top layer of Fe powder. During
the sintering operation the copper foil will form a bond with the
Fe powder. The bond can be further enhanced by allowing a burr to
form on the edge of the copper during the punching operation and
the burr will help to anchor the Cu into the iron powder. An
alternative method of forming the copper bonding layer is to add a
layer of Copper powder on top of the hot end Fe layer. If this
method is used it is extremely important to not allow any Cu dust
to mix with the PbTe or to react with the Mo die. Good mold release
agents such as WS.sub.2 will help to prevent this reaction.
Dual Egg-Crate Construction
[0059] An eggcrate is used to: [0060] fix each leg in the desired
location, [0061] allow the formation of electrical connectors by
thermal spray metallizing the surface of the loaded eggcrate and
then sanding down the surface of the deposited metal until the
eggcrate wall is exposed [0062] prevent vapor phase transport of
the Te and/or PbTe from the P leg to the N leg or vice versa.
[0063] For applications that do not exceed 300.degree. C. an
eggcrate can be inexpensively fabricated from high temperature
polymers such as liquid crystal polymers or polyimide but for
temperatures exceeding 300.degree. C. the eggcrate must be
fabricated from glasses or ceramics. Ceramic eggcrates are
discussed in earlier Hi-Z patents but one low cost variation to a
ceramic eggcrate or a two part polymer/ceramic eggcrate is to use a
polymer eggcrate on the cold side of the module to locate the
thermoelectric elements and to allow the formation of the cold side
conductors by metallizing the module and then fill the hot space
between the hot side of the legs with a castable ceramic. The
castable ceramic will help support and strengthen the legs and it
will prevent transport of the Te and/or PbTe from one leg to the
other. The hot side conductors can then be formed by metallizing
through a mask or through some other process.
[0064] An alternative to using ceramics to form the hot side of the
two part egg-crate is to use mica to form the hot side egg-crate.
This could be done in a manner similar to the egg-crate discussed
in the background section of this specification but only the hot
half of the egg-crate is formed using that process and the cold
side of the egg-crate is formed using and injection molded polymer
such as Zenite.
[0065] In preferred embodiments, the high-temperature ceramic
(preferably Resbond 989 or Resbond 908) available as a
high-temperature, general purpose ceramic adhesive from Cotronix
Corporation, and the liquid crystal polymer material (preferably
Zenite) available from DuPont in the form of a liquid crystal
polymer resin.
Bonding Copper Hot Side Conductors
[0066] Copper conductors are bonded to the copper bonding layer on
the hot end of the legs by fixing the copper conductors to the hot
side module in the desired location using an adhesive that will
vaporize away at temperatures above 300.degree. C. The module with
the hot side conductors is then placed in between a heater and a
chilled plate under a force of 500 psi in a chamber where the
atmosphere can be controlled. In a reducing atmosphere the hot side
of the module is heated to 575.degree. C. and held until the
resistance of the module drops to below 500 m.OMEGA.. This usually
happens in less than two hours.
Alternative P Leg
[0067] In the above description of the first preferred embodiment
the cold side of the P leg is made from a Bi.sub.2Te.sub.3 alloy
and the hot side of the leg is made from a PbTe alloy. As described
in Table 2 a layer of PbTe powder is placed in the die and a layer
of Bi.sub.2Te.sub.3 powder is placed on top of the PbTe powder and
the two powders are sintered in a vacuum at 500.degree. C. under a
pressure of 6,000 psi. Pressing the Bi.sub.2Te.sub.3 powder in this
manner will cause the grains to orient in a direction that will
yield non-optimum properties for the Bi.sub.2Te.sub.3 portion of
the leg. However, the properties of the Bi.sub.2Te.sub.3 portion
are still far superior to a P leg as compared to using only PbTe
for the cold portion of the leg.
[0068] A Bi.sub.2Te.sub.3 segment with better properties can be
obtained by pressing a B.sub.2Te.sub.3 leg in a separate step by
cutting a leg segment from a cast Bridgeman ingot in such a way
that the ideal properties are oriented to be parallel with the
pressing direction and then placing PbTe powder on top of the
prefabricated Bi.sub.2Te.sub.3 segment. The Bi.sub.2Te.sub.3 leg
segment must be a snug fit with the die to prevent the grains from
rotating during the vacuum hot pressing operation.
Lubricant
[0069] When vacuum hot pressing PbTe legs it is important to use a
lubricant to: [0070] prevent the components of the thermoelectric
leg from reacting with the die. [0071] allow the part to be easily
removed from the die.
[0072] Some of the better mold release agents are MoS.sub.2 and
WS.sub.2.
Hermetic Seal
[0073] In order for a thermoelectric module containing PbTe to last
more than a few thousand hours it must be contained in a reducing
environment with a hermetic seal that will prevent transmission of
any oxygen through the seal. A hermetic seal of this quality can
not be obtained using organic materials and must be made with
metals, ceramics or glasses. One method of doing this is to thermal
spray the entire module with an insulating material such as alumina
and then to spray a layer of stainless steel on top of the ceramic.
The metal coating will contain small pinholes that can then be
sealed by melting the metal surface with a laser. The space inside
the module can be made to be reducing by dispersing silicon
particles inside the castable ceramic.
[0074] An alternative method that has been shown to be promising is
to powder coat the module with a high temperature powder coating
paint such as Eastman's HotCoat High-Temp powders. These paints are
based on silicone and when exposed to high temperatures the
silicone is converted to glass. Several layers may be necessary to
obtain a complete seal. Other forms of high temperature paints
could be used by powder coating paints can obtain a more uniform
coating with better integrity.
[0075] To further improve the seal created by the powder coated
paint an additional metal layer can be added by thermal spraying or
by electroplating and/or electroforming.
Technique to Prevent Te Evaporation and Contamination
[0076] As indicated in the background section life testing of PbTe
modules by Applicants has shown that some degradation of the module
occurs after approximately 1,000 hours of operation. Applicants
have discovered that the degradation can be attributed to
"cross-talk" between the N legs and the P legs near the hot
junction caused by evaporation of tellurium from the P leg
contaminating the N leg. (As explained in the background section an
excess of lead in the N leg is what provides the n-leg with some of
its thermoelectric doping properties and an excess of Te in the P
leg provides the P leg with some of its thermoelectricity.) The
problem is prevented in preferred embodiments with two techniques:
First, the egg-crate walls separating the N legs from the P legs
may be extended to contact the hot conductor so that tellurium
vapor is restrained from migrating to the n-leg. A second technique
used by Applicants is to add a thin layer of PbSnMnTe at the top
(hot side) of the p-legs (not shown in the drawings). Applicants
have determined that elemental tellurium exhibits little or no
evaporation from PbSnMnTe. While the PbSnMnTe material does not
have as good thermoelectric properties as PbTe, the amount used is
small, only 0.020 inch long out of 0.450 inch overall length. The
PbSnMnTe segments will be vacuum hot-pressed and sintered with the
2P type PbTe and Bi.sub.2Te.sub.3. In some embodiments the PbSnMnTe
material may be substituted for the hot portion of the p-legs.
[0077] With the use of a hot side egg-crate the hot side conductors
can be formed using thermal spray metallization. Thermal spraying
the hot side conductors does a good job of sealing the hot side of
the legs thereby hindering the diffusion of Te vapors from one
cavity to another.
Techniques to Better Orient the Bi.sub.2Te.sub.3 Segments
[0078] Some other techniques to better orient the Bi.sub.2Te.sub.3
Segments are described below:
Press, Separate and Bond
[0079] This method consists of the following steps: [0080] 1) Press
and sinter the PbTe segment. [0081] 2) Press and sinter the
Bi.sub.2Te.sub.3 segment. [0082] 3) Rotate the Bi.sub.2Te.sub.3
segment so that it's best properties (perpendicular to the pressing
direction) are in the correct orientation and insert the segment
into a tight fitting die. If the die allows the Bi.sub.2Te.sub.3
segment to deform an excessive amount the grains will rotate and
the properties will be degraded. [0083] 4) Insert the PbTe segment
so it rests snuggly against the Bi.sub.2Te.sub.3 segment. It may be
useful to use an interface layer to aid in the bonding. One
potential interface layer is SnTe. [0084] 5) Sinter the combined
segments at 500.degree. C. in a reducing atmosphere for 48 hours.
[0085] 1) ing atmosphere for 48 hours. [0086] And alternative to
this method would be to: [0087] 1) Press and sinter the
Bi.sub.2Te.sub.3 segment. [0088] 2) Rotate the Bi.sub.2Te.sub.3
segment so that it's best properties (perpendicular to the pressing
direction) are in the correct orientation and insert the segment
into a tight fitting die. If the die allows the Bi.sub.2Te.sub.3
segment to deform an excessive amount, the grains will rotate and
the electrical properties will be degraded. [0089] 3) Fill the
remainder of the die with PbTe powder. An interface layer such as
SnTe may be useful. [0090] 4) Cold press and sinter the resultant
element or the element could be hot pressed.
Press, Separate and then Co-Press
[0091] During the pressing operation, N type Bi.sub.2Te.sub.3
grains become oriented in the plane perpendicular to the pressing
direction. To make a useful pressed N type Bi.sub.2Te.sub.3 leg,
the leg must be used so that the temperature gradient is
perpendicular to the pressing direction of the element. While this
is simple to do with an un-segmented leg it is difficult to do this
with a segmented leg because the powders from the two segments will
tend to mix in the die and an accurate segment line will be
difficult to achieve. The disclosed method consists of pressing the
two segments into low density blocks that contain the proper amount
of material for the desired final segments and then inserting these
pre-pressed blocks into a die that will be subsequently pressed
perpendicular to the expected temperature gradient. The
Bi.sub.2Te.sub.3 segment and the PbTe segment may be two separate
pieces as shown in FIG. 9, or a single piece as shown in FIG. 10
and as described below: [0092] 1) Separately cold press the PbTe
and the Bi.sub.2Te.sub.3 segments. The pellets should be of low
density and each segment should have the proper amount of material
for the final leg. The two segments should be of a geometry small
enough to fit into the die that will be used for the final press.
[0093] 2) Place the segments (either one piece or two pieces)
side-by-side into a die that has the desired final geometry. [0094]
3) Press the pellet to the desired density. Because the pellet has
a low density and because the die is larger than the pellet, the
pellet will under go significant deformation during this second
pressing operation. As the Bi.sub.2Te.sub.3 segment is pressed the
movement caused by the punch will cause the Bi.sub.2Te.sub.3 grains
to rotate into the desired orientation and result in optimum
properties in the same direction as the intended temperature
gradient. The die must be close to the same size as the pellet
being pressed or the segment line will be too distorted. [0095] 4)
Sinter the combined segments elements at 500.degree. C. for 48
hours.
Three Segment P-Legs and Two Segment N-Legs
[0096] A preferred alternative of the present invention is similar
to the first preferred embodiment. It combines the above techniques
with existing state of the art materials to produce a cost
effective thermoelectric module with an accumulated efficiency of
16 percent when operated between a hot side temperature of about
560.degree. C. and 50.degree. C. The stack-up of improved
efficiencies is shown in Table 3. Conventional available PbTe
materials can provide modules with efficiencies of 7 percent with
the above temperature difference. In this preferred embodiment
Applicants increase the efficiency to 9 percent by adding a P-type
Bi.sub.2Te.sub.a cold segment and further increase the efficiency
to 10 percent by adding an N-type cold segment using a new
Bi.sub.2Te.sub.3 material developed by Applicants. The efficiency
is further increased to 11 percent by dividing the PbTe material
into two segments, i.e. 2P and 3P. An improved PbTe material is
used to gain another incremental improvement in efficiency to 12
percent as described in U.S. patent application Ser. No. 12/293,170
which is incorporated by reference herein. Utilizing nano-grained
Bi.sub.2Te.sub.3 available from GMZ Inc., with offices in Waltham,
Mass., provides another 1.0 percent to increase the accumulated
efficiency to 13 percent and finally an additional 3 percent
improvement is provided by use of nano-grained PbTe material to
provide a module that operates at efficiencies of about 16
percent.
TABLE-US-00005 TABLE 3 Approaches to Improving Module Efficiency
Technology Digit Increase Accum. Approach status in efficiency
Efficiency Commercially Hi-Z fabricates -- 7% available PbTe P type
Bi.sub.2Te.sub.3 existing 2% 9% cold segment N type
Bi.sub.2Te.sub.3 new 1% 10% cold segment 2P with 3P hot segment
new/existing 1% 11% Vendor X, N type PbTe new 1% 12% Nanograined
Bi.sub.2Te.sub.3 existing 1% 13% from GMZ Inc. Nanograined PbTe
legs new 3% 16%
Alternate Bulk Alloys
[0097] Lead telluride based alloys have been used since the 1960s
and the alloys and recommended doping levels were documented above.
Their thermoelectric properties versus temperature are given in
many publications such as Chapter 10, "Lead Telluride Alloys and
Junctions" of Thermoelectric Materials and Devices, Cadoff and
Miller, published by Reinhold Publishing Corporation of New
York.
[0098] In the past four years newer PbTe based alloys have evolved
that have better properties than the conventional PbTe based alloys
noted above. For example a Jul. 25, 2008 article in Science Daily
reported on a lead telluride material developed at Ohio State
University having substantial improvements in efficiency over prior
art lead telluride materials. This new material is doped with
thallium instead of sodium. The article suggests that the
efficiency of the new material may be twice the efficiency of prior
art lead telluride. Other experimenters have developed a new N type
PbTe which is doped with Ti and iodine and has a ZT of 1.7 (PbTe is
typically about 1.0). The P type alloy is Pb.sub.7Te.sub.3 and
doped with AgTe. While it has the same ZT as PbTe 2P its advantage
is that it can be used with Fe hot shoes segmented to
Bi.sub.2Te.sub.3 alloys.
[0099] Applicants are seeking to prepare bulk lead telluride
thermoelectric material with a finer grain size than has previously
been achieved. Fine grain size is expected to lower the thermal
conductivity of the material without significant impact on
resistivity or Seebeck coefficient, thus raising its ZT and
efficiency. In previous attempts others have made to produce a
fine-grained PbTe, the grain size was observed to coarsen rapidly,
even near room temperature, so the benefit of small grain size
could not be retained. In this study, Applicants seek to preserve a
fine grain structure by additions of very fine alumina powder,
which is expected to produce a grain boundary pinning effect, thus
stabilizing the fine grain size. Applicants' recent results
indicate that the PbTe grain size can indeed be held below 2 .mu.m,
even with processing at 800.degree. C.
Lead Telluride Only Modules
[0100] Some of the techniques described herein can be utilized in
modules where the entire legs are comprised of only lead telluride
thermoelectric alloys. Preferably, the lead telluride alloy or
alloys are one or more of the newer very high efficient alloys.
Other Thermoelectric Materials
[0101] The methods described above for making the first preferred
embodiment can be used with respect of many other thermoelectric
materials several of which provide better performance especially at
the high temperature ranges. So these materials can be stacked and
bonded using the above techniques to improve performance. Some of
these materials are:
TABLE-US-00006 Alloy Peak Z Temperature at Peak Z (k) TAGS 0.0020
675 Si80Ge20 0.0008 800 LaTe 0.0010 925 PbTe 0.0018 775 Zintle
0.0011 1,250 Bi2Te3 0.0025 325 Skkutterudite 0.0018 925 FeSi2
0.0002 925
[0102] Since these materials are all strongly temperature dependant
they should be combined in such a way that each would operate in
its ideal temperature range. For example a good combination would
be bismuth telluride at cold temperatures of about 325 K, followed
by a segment of TAGS at about 675 K followed by lead telluride at
about 775 K and finally a hot side skutterudite segment operating
at about 925 K. As described above interface layers could be used
to accommodate differences in thermal expansion and additionally
interface layers could also be used as diffusion barriers to
inhibit reactions between the various layers.
Generator Design Using PbTe-Type Modules of the Preferred
Embodiment
[0103] The high-temperature module of the preferred embodiment
requires encapsulation to prevent oxidation of the N and P alloys
with an accompanying decrease in thermoelectric properties. An
example of encapsulating PbTe modules would be the 1 kW generator
for diesel trucks shown at 16 in FIG. 5 since all of the modules
are encapsulated together encapsulation of the individual modules
is not necessary. In this example the generator is attached to a
5-inch diameter exhaust pipe 18. Lead telluride thermoelectric
modules 20 are mounted on a support structure 22 which is machined
or formed to create an octagon. The inside surface is generally
round with fins (not shown) which protrude into the gas stream to
provide a greater heat transfer area. The basic design is similar
to the design described in U.S. Pat. No. 5,625,245 which is hereby
incorporated herein by reference.
[0104] The casting of the support structure has two flanges 24 and
26, one large and one small which are perpendicular to the main
part of the support structure. The large flange 24 is about 10
inches in diameter while the small flange 26 is about 8 inches in
diameter.
[0105] The large flange contains feed-throughs for both the two
electrical connections and four water connections. The two electric
feed-throughs 28 are electrically isolated with alumina insulators
from the support structure. Both the large and small flange will
contain a weld preparation so a metal dust cover 30 can be welded
in position.
[0106] The four water feed-through elements 32 consists of one inch
diameter tubes that are welded into the flange. The inside portion
of the water tubes are welded to a wire reinforced metal bellows
hose with the other end connected to the heat sink by a compression
fitting or a stainless to aluminum bimetallic joint. Two of the
tubes are inlets and the other two tubes are outlets for the
cooling water.
[0107] Once the generator is assembled and the flanges between the
support structure and the dust cover are welded as shown at 34, the
interior volume will be evacuated and back filled with an inert gas
such as Argon through a small 3/8 inch diameter tube 36 in the dust
cover to about 75% of one atmosphere when at normal room
temperature (.about.20.degree. c.). Once filled, the fill tube will
be pinched off and welded.
[0108] In a preferred embodiment nine thermoelectric modules 20 of
the first preferred embodiment are mounted on each of the eight
sides of the hexagonal structure for a total of 96 modules.
Applicants estimate a total electrical output of about 1.1
kilowatts. This estimate is based on prior performance with the
structure described in the U.S. Pat. No. 5,625,245, utilizing
modules of the first preferred embodiment and assuming an exhaust
hot side temperature of about 550.degree. C. and cold side cooling
water temperature of about 100.degree. C.
Module Encapsulation
[0109] An alternative to encapsulation of the entire generator, as
shown in FIG. 5A and FIG. 11, is to encapsulate the individual
modules. FIG. 5B shows such a technique. This is an example where
the module is encapsulated in a thin metal capsule which is
comprised of a bottom plate and a cover. The two parts are welded
at the seam. The metal capsule requires a thin insulating sheet on
both the hot side and the cold side. Capsules can also be formed
with insulating material such as SiO.sub.2.
Molded Egg-Crate with Legs in Place
[0110] An alternative technique for making the thermoelectric
module of the present invention is molding the egg-crate with the
thermoelectric legs in place.
[0111] Thermoelectric egg-crates serve several functions. They hold
the elements in the correct locations, they define the pattern of
the cold side connectors and they locate the hot side connectors.
Egg-crates can be assembled from mica as described in U.S. Pat. No.
4,611,089, injection molded plastic (gapless egg-crate) as
described in U.S. Pat. No. 5,875,098 or injection molded ceramic or
injection molded plastic and ceramic as described in parent
application Ser. No. 12/317,170. An alternative method of
fabricating the egg-crate is proposed below: [0112] 1) A two-part
mold is fabricated from a suitable material. Some possible mold
materials are polyethylene, aluminum or Teflon. The mold is
designed to hold the thermoelectric elements in place while a mold
material is poured around the elements. [0113] 2) Thermoelectric
elements are loaded into the top half of the mold assuring that the
N and P elements are located appropriately as shown in FIG. 11A.
[0114] 3) The bottom half of the mold is put in place and castable
material is poured into the mold filling the spaces labeled "cast
material". Several choices are available for suitable castable
materials. A two part epoxy would be suitable for low temperature
applications. For high temperature applications Aremco's Ceramacast
584 or 645-N would be a good choice. [0115] 4) After the mold
material has cured the part is removed from the mold. [0116] 5) The
cast egg-crate containing the thermoelectric elements is then ready
to have the cold side contacts made as described in U.S. Pat. No.
5,875,098. The loaded egg-crate is then fixtured appropriately to
hold the elements in place while aluminum with a proper molybdenum
bond coat is thermally sprayed onto the cold (bismuth telluride)
side of the module. This process is explained in detail in U.S.
Pat. No. 4,611,089. An alternative metal to Mo/Al combination is
zinc.
[0117] Unlike the gapless egg-crate modules described in U.S. Pat.
No. 5,875,098, only the cold side of the segmented module is
connected in this manner. With the module held firmly to prevent
warping, the deposited coating (Mo, Al or zinc) is sanded to expose
the egg-crate walls which thereby define the cold side electrical
connectors as described in U.S. Pat. No. 5,875,098. The module is
then lapped to a suitable finish and the bottom (cold side). Copper
shoes provide the hot side electrical connections.
[0118] Using a slightly different mold design, a high temperature
mold material could be used for the hot side of the module and a
low thermal conductivity material could be used for the cold side
as suggested by the dashed line in FIG. 11D. Ideally, a two part
egg-crate with the cold side being a polymer/organic based material
for strength and the hot side being a ceramic based material for
high temperature resistance would be the best choice.
Preferred Module
[0119] An example of a module that incorporates the features
described above will have the following properties:
TABLE-US-00007 Module width 5.0 cm Module depth 5.0 cm Module
thickness 1.0 cm Hot side temperature 560.degree. C. Cold side
Temperature 60.degree. C.
[0120] The thermoelectric legs used in the preferred embodiment are
7.0 mm long but only 5 mm of that length is active thermoelectric
material. Each end of the leg, with the exception of the cold side
of the P leg, will consist of 0.5 mm of Fe and 0.5 mm of a
transition material consisting of 25% Fe and 75% PbTe. The cold
side of the P leg will consist of 1 mm of 90% Fe and 10%
Bi.sub.2Te.sub.3.
[0121] The 32 P type legs--each P leg is 7.0 mm long, 5.0 mm wide
and 5.0 mm deep. The bottom (cold side) 2.2 mm is bismuth telluride
and the top 2.8 mm is 2P.
[0122] The 32 N type legs--each N leg is 7.0 mm long, 5.0 mm wide
and 5.0 mm deep. The bottom (cold side) 3.8 mm is 2N and the top
1.2 mm is 3N.
[0123] Knowing the thermal conductivity of the thermoelectric
alloys and how it changes with temperature gradient along the
length of the leg is calculated and the segment line for each
thermoelectric alloy is positioned as indicated in FIGS. 6A and 6B.
According to these figures the Bi.sub.2Te.sub.3 segment on the N
leg would be positioned so that its center is at 300.degree. C. The
3N segment on the hot side on the N leg should be positioned so
that its center is at 420.degree. C. The 3P segment on the hot side
of the P leg is there to avoid Te vaporization so it should be made
as thin as possible and still form a reliable separation between
the 2P material and the iron shoe. Preferably that thickness is
about 1 mm.
[0124] Performance specifications are as follows:
TABLE-US-00008 Power 27 watts Open circuit voltage 6.0 volts
Voltage at matched load 3.0 volts Heat flux 15 W/cm.sup.2 Internal
resistance 0.33 ohms Efficiency (max) 10%
Variations
[0125] While the above description contains many specificities, the
reader should not construe these as limitations on the scope of the
invention, but merely as exemplifications of preferred embodiments
thereof. For example:
Other High Temperature Thermoelectric Alloys
[0126] Some of the other thermoelectric alloys that are attractive
over high-temperature ranges are: [0127] Si-20% Ge, LaTe.sub.1.4
type alloys [0128] Zintl, (Yb.sub.14MnSb.sub.11) [0129] TAGS
(AgSbTe.sub.2).sub.0.15(GeTe).sub.0.85 [0130] The skutterudites
such as CoSb.sub.4 type alloys [0131] The half-Huesler alloys
The LAST and FAST Alloys of Michigan State University
[0132] All of these bulk alloys and others under development can be
used in the new ZrO2/Zenite egg-crate design described in the
parent applications that has been incorporated by reference herein
or the other eggcrate designs that are described herein.
Other Fabrication Techniques
Segmented Legs
[0133] Separate portions of the segmented legs can be readily
bonded together by passing a current through both using a spot
welding machine sometimes also referred to as spark sintering. As
the current passes through the samples, the interface, which is
purposely made to have a high resistance, reach a temperature at
which bonding takes place. Sometimes a liquid phase is formed. The
spot welding time is only a fraction of a second. To form a
consistent bond, wire mesh has been used. The mesh preferentially
heats up and imbeds itself in both materials. The N type
Bi.sub.2Te.sub.3, which must be used in the correct orientation,
retained its crystalline orientation and was successfully bonded to
N type PbTe. The contact resistance between the two components was
less than 100 .mu..OMEGA.-cm.sup.2 and the bond was strong. This
bonding technique was also successful for bonding the P leg PbTe
and Bi.sub.2Te.sub.3 segments.
[0134] The PbTe portions of the p-legs can also be cold pressed and
sintered separately from the Bi.sub.2Te.sub.3 portions. When they
are subject to hot operating conditions they will diffusion bond.
The same applies to the n-legs.
Hot Pressing of the Legs
[0135] Another option is to hot press the thermoelectric materials
in bulk then slice and dice them into legs.
Tape Casting
[0136] An alternative method to forming the legs is to cast each of
the desired materials into a fugitive binder and form thin sheets
of the material called "tape". In this manner a large die can be
filled with various stacked tapes containing the desired materials.
The tape is filled with a known amount of the desired powder so
that when the tape is heated and pressed in a vacuum the binder is
driven off and the correct amount of powder remains behind to form
a layer of the material that has the desired thickness. For example
a large die could be filled with first a tape containing Fe powder
and then a tape containing mixed Fe and PbTe powders with about 75
weight percent PbTe and 25% Fe and then one or more tapes
containing PbTe powder and then a second layer of mixed Fe and PbTe
powders and finally a second layer of tape containing Fe powder.
The entire stack up of tapes are then forced in a Mo die and
pressed to 6,000 psi while in a vacuum and heated to 600.degree. C.
for one hour. The Fe and mixed powders would each contain enough
powder to result in a layer about 0.5 mm thick and the PbTe
layer(s) would result in about 5 mm of PbTe. The final product will
be a slab of material about 7 mm thick. The slab is then cut into
rectangles that are 5 mm square that have the original thickness of
the slab.
[0137] Important to the success of this approach is selection of
the fugitive binder. The only products that we know will work are a
line of products based on the polymerization of carbon dioxide
called QPAC.TM. made by Empower Materials, Inc. and especially a
particular product called "QPAC-40 Low Monomer". This material can
be dissolved in acetone to form a viscous sticky liquid. The
powdered materials comprising individual segments or layers of the
TE leg can be mixed with this liquid and cast into 2-dimensional
forms. When the acetone evaporates, the resulting material is a
self-cohesive and flexible solid sheet or "tape". The critical
characteristic of the QPAC is that when heated in vacuum or inert
atmosphere, it will thermally decompose below 325.degree. C. and
turn completely into carbon dioxide gas, leaving no detectable
solid residue intermixed with the powder. The fugitive binder thus
functions as a temporary carrier that enables powders of multiple
different compositions to be arranged in particular fixed spatial
arrangement or configuration within the compaction die until enough
pressure can be applied to permanently fix them into their intended
final relative positions.
[0138] The process called "tape casting" is well known in the field
of electronic ceramics, but it is nearly always practiced with
materials that can be sintered in air, such as oxide ceramics like
alumina. In air-firing systems, acrylic polymers are most commonly
chosen binders since they burn away in air leaving negible
residues.
Other Crate Designs
Aerogel Crate
[0139] In one variation, an aerogel material is used to fill all
unoccupied spaces in the egg-crate. A module assembly will be sent
to an aerogel fabrication laboratory. They will immerse it in
silica sol immediately after dropping the pH of the sol. The sol
converts to silica gel over the next few days. It is then subjected
to a supercritical drying process of tightly controlled temperature
and pressure condition in a bath of supercritical liquid CO.sub.2.
This process removes all the water in the gel and replaces it with
gaseous CO.sub.2. The Aerogel serves multiple functions: (1)
helping to hold the module together, (2) reducing the sublimation
rate of the PbTe and (3) providing thermal and electrical
insulation around the legs.
[0140] In another variation, an egg-crate made of non-woven
refractory oxide fiber material, possibly with a fugitive polymer
binder, and having the consistency of stiff paper or card stock is
used. After assembly, the binder is burned away, leaving a porous
fiber structure that is them infiltrated with Aerogel. The fiber
reinforcement of the aerogel gives added strength and
toughness.
[0141] Those skilled in the art will envision many other possible
variations within its scope. Accordingly, the reader is requested
to determine the scope of the invention by the appended claims and
their legal equivalents, and not by the examples which have been
given.
* * * * *